Mod to Keep Radio ON After Shutting Ignition OFF

Ok peeps, buckle up! This might get bumpy.

Don't blame me if you try this and blow your truck up, this is for informational purposes only. If you mess with your wiring, you do so at your own risk!

One thing that frankly has rubbed me the wrong way ever since buying my new Ridgeline about 4 months ago is that the radio instantly shuts off when you turn the truck off. Today I had the day off work and decided to see if I could "fix" that oversight. Ideally, the radio (or for that matter the entire accessory circuit) would stay on until a door was opened, just like all other cars I've owned for the last 20+ years. I'm super thrilled to report that with an incredibly simple wiring mod, my Ridgeline now does exactly that! No cutting into a factory wiring harness. No soldering. Just two "add-a-fuse" adapters and some fuses.

First, the accessory circuit. After a lot of probing around under the dash I found that there's a 7.5A fuse that controls the accessory circuit. Specifically it's fuse 40 in the big fuse block just behind the parking brake foot lever. My guess is that this fuse must protect a relay because 7.5A isn't enough to power the cigarette lighter alone, not to mention the radio, amplifier, USB ports, and whatever else is on in accessory mode. This fuse is powered in accessory and run mode, but instantly shuts off when the truck is shut off. Back-feeding voltage from the battery through a small fuse and onto this 7.5A fuse did indeed power up the accessory circuit with the truck off. Great! I'm halfway there.

Next, I pondered installing a separate time delay relay to get separate delayed power. But I knew that there are other circuits that already have a time delay in the Ridgeline. My clue here was that the power windows do exactly what I wanted the radio to do - they keep working until a door is opened. But, oddly, the windows do NOT work in accessory mode; only run mode and thereafter until a door is opened. So after more probing, I found that there are 5 fuses for the power windows on trucks with sliding rear glass (1, 4, 11, 12, 13). My guess is that non-sliding rear glass trucks will have 4 fuses (1, 4, 11, 13). Oddly, the driver's window fuse (fuse 1) is always on. But the others are only powered when the truck is in run mode, or for a while thereafter until a door is opened. PERFECT!

Here's the meat-and-potatoes part.

I just took two "Add-A-Fuse" circuit adapters and populated them so that the primary circuit is fused accordingly. So the 20A power window fuse is still fused at 20A. The 7.5A accessory circuit is fused at 7.5A. Then I put a 7.5A fuse in the power window "added" circuit and routed it over to the accessory "added" circuit. I plugged the 20A Add-A-Fuse adapter into the rear sliding window fuse spot (fuse 12) and the 7.5A Add-A-Fuse adapter into the Accessory fuse spot (fuse 40). Be aware that there's a correct orientation to these Add-A-Fuse adapters so that the added circuit is fused and not directly connected.

Done.

Now the Ridgeline stays in accessory mode for almost exactly 10 minutes after shutting it off, unless you open a door which instantly shuts off accessory mode. The cigarette lighter style power port continues to work, too. Interestingly after shutting off the truck, the dash now displays "Accessory Mode" until a door is opened.

The unintended consequence of this modification is that the power sliding rear window now works in accessory mode. I chose that window because it draws by far the least current and that 7.5A fuse seems to work fine with it, even though it's factory fused at 20A. It would be very interesting to know if on trucks without the power sliding rear window if this would still work exactly the same way. In other words, there may still be a time delayed terminal in the fuse block at the empty fuse 12 spot that would work perfectly for this.

Here's a picture of the fuse block with both Add-A-Fuse adapters installed and a Wago lever nut connecting the added circuit from each. Fuse 12 is at the bottom right and fuse 40 is along the top row.

Note that the top Add-A-Fuse is technically installed upside down meaning that it's added circuit is NOT fused as it should be normally (technically it's double-fused I guess). But that's ok because I'm feeding it from a fused circuit already. The reason it's upside down is because it won't fit the other way because the relays above it interfere. Oh, and these Add-A-Fuse adapters are "ATM - Miniature", not the correct "ATM LP - Miniature Low Profile". I couldn't find any correct low profile Add-A-Fuse adapters today so I'll have to order some, just to keep the fuses matching. But these ATM adapters fit just fine in the ATM LP sockets.

The sticker right next to the fuse block:

I've only had this mod installed for a short time today, but I was excited to report to the group. I'll keep it installed and obviously update if there are any unexpected issues. The only issues that come to mind are that:
1) When in accessory mode, I'm feeding the 20A sliding rear window circuit off of a 7.5A fuse. So I risk blowing that fuse. But the rear window draws a lot less current than the door windows, so I think I'm pretty safe there.
2) I have TWO 7.5A fuses feeding the fuse 40 accessory circuit. But like I said earlier, my guess is that it might just be feeding the coil to a relay. If anyone has a schematic for what this fuse feeds, I would appreciate the info. If it's truly just a relay coil, then I could drop those two fuses down to 3A.