BMW Radar Detector Hardwire

Well, this post has nothing to do with photography (although that shot above is a shot I took of my new vehicle), but I thought I'd post some shots and description of how I hardwired a Cobra radar detector into my BMW 335i Coupe.

One of the problems with simply plugging a radar detector into most cars, and especially into the bimmer, is that it necessitates hanging some ugly cord across the dashboard and into a cigarette lighter socket. Such an installation creates several layers of hideousness: the inelegant cord, the obstruction of various controls and displays, and some compartment hanging open with a lighter plug protruding from it... not exactly what the car designers had in mind.

Hardwiring it in—while improving the aesthetics—creates its own challenges. I especially did not want to do any warranty-voiding mods on my new ride. Here's one solution, inspired by this guy's approach.

Conveniently, the 2009 BMW 335i Coupe (E92) has the fusebox right behind the glovebox. This is where we'll tap into an unused circuit. The back of the glovebox is held in place by two simple quarter-turn knobs and easily pops off to expose the fusebox. The only challenge now is to find an empty fuse slot that is powered when the cars is turned on, but not when the car is turned off (to avoid draining the battery, obviously).

Trial and error with a multitestor (volt-ohm meter) will identify an available circuit. In the case of my car, fuse slot #5 fit the bill. In case you're interested, there's a little fold-out diagram attached to the back of the glovebox panel that shows what each fuse is intended for.

Now comes the important part: to tap into the circuit, get ahold of a part known as an Add-A-Circuit. They come in both ATC (full size) and ATM (mini size), so for fuse #5 I bought the mini (ATM) version. I found mine at AutoZone for about $6. This little part plugs into the fusebox and has a slot for a fuse and a positive lead extending from it to which you can add any 12 volt device... in our case, the radar detector.My Add-A-Circuit is made by Bussman, but Littelfuse (yes, that's spelled correctly) also makes one. I added a 10 amp fuse since that's what I had on hand, but fewer amps would have been sufficient for a radar detector.

I didn't want to cut into my radar detector's cord either, so I ended up cannibalizing a lighter socket extension cord from RadioShack that I had on hand to create a lighter socket in the glovebox. FWIW, the positive lead is the tip of the socket and the ground is the ring of the socket.

For the ground connection, I crimped and soldered a ring connector to the negative lead from the lighter socket. For ease of attachment, I cut out part of the ring as shown in the photo so that I could just loosen the grounding screw rather than remove it completely. The grounding screw takes a #27 TORX bit (maybe a #30 Torx would work, too, but I didn't try it since the #27 was adequate).

With the Add-A-Circuit wired up, it was simply a matter of plugging in the Add-A-Circuit into fuse slot #5, screwing down the grounding bolt to the ring connector, and bringing the attached cigarette lighter socket forward into the glove box (see photo).

After that, I replaced the glovebox panel, plugged in the radar detector's power cable, and passed the cable out through the back of the glovebox toward the base of the right A-pillar.

The cord runs in the weather stripping around the door, over the right edge of the dash and is tucked into the gap where the windshield meets the dash board. The following pictures show it in place (along with a 20GB iPod, of course).

All in all, a very inexpensive project that requires minimal skills and minimal time and yet gives a much cleaner look to the passenger compartment: no cables dangling about and I can keep the front console's ashtray/cigarette lighter bay
closed.