So today I finally decided to relocate the electronics in my trunk in the spare tire wheel well. Given that I have an earlier model E60 with PDC, the only two things I had to move were the micro power module (MPM) and a small, mint green relay.
For those of you who don't know much about the E60, the electronics located in the spare tire wheel well are very prone to water damage. Whether it be a clogged sunroof drain, poor seal around the taillights (especially if you swap in aftermarket taillights), or even internally spilled liquids; dealerships will charge upwards of $1000 to fix the electronics.
I had done a bit of research in the past about people replacing the electronics themselves. It seems that for about $200 for a MPM and $10 for a relay, I could replace all the electronics myself if I had to - however its much cheaper to just buy some adhesive backed velcro and move them yourself.
For this DIY you'll need a small flat head screw driver to pry some plastic covers off. A medium sized phillips screw driver to remove some screws (crazy huh?). Some adhesive backed velco to hold the electronic modules in their new resting places. Optionally, you'll need a couple cables ties and something to cut them with.
First, you'll need to open up the trunk. As you can see mine is quite cluttered...
Next, clear everything out in the center and latch the floor using the attached hook as shown.
Remove the spare tire by loosening the center wing nut and lifting it out. You may need to remove the spare tire jack first if its in the way. After this, you should be able to see the wires running to the electronics in the spare wheel well. Note the direction of the arrow (pointing towards the front of the car) for reassembly.
Now, there are two insulating pieces remaining in which the electronics are located. Simply pull the top half off and now you should have access to all of the electronics.
In my car, the MPM is located towards the top of the picture and relay is located towards the bottom. If you have PDC or other fancy communications options there may be more modules here.
Now, we need to loosen up the plastic interior trunk trim. It is not necessary to remove the entire piece of trim itself as we really just need to re-route some wires.
There are two screws we need to loosen using a medium-sized phillips screwdriver. One in the center which is covered by small piece of plastic.
I used a small flat head screwdriver to pry the cover off, but you may be able to get it out by hand if you haven't clipped your fingernails in the past week.
The other screw is located near driver side cargo hook anchor.
Using the same small flat head screw driver, pry the plastic cap off to expose the screw itself. Note that this screw and the center one are identical so don't worry about mixing them up. Note that picture shown is for passenger side screw. You do not need to remove the screw. It was just easier for me to get the picture on this side.
Now with both the screws removed, we need to remove the interior trunk trim covering the tail light on the driver side. This is quite simple. Find the two large plastic screws such as the one shown below.
Now rotate both of the screws (should be able to do this by hand) so that the slot is aligned vertically. Once the slot is aligned vertically, you should be able to simply pull the plastic screw straight out.
After removing both plastic screws, place your hand in the small rectangular cutout near the bottom of the piece of trim and simply rotate the back end of the piece of trim towards the center of the trunk. The trim should simply slide free at which point you can lift it out and set it aside.
Now, there is another piece of insulation just to side of the spare wheel well which we need to remove. Simply lift up towards the interlocked seem and pull up. The piece of insulation should come free and be able to easily be removed.
Next, remove the electronic modules from their current position and place them somewhere on the driver side of the trunk where the trim was removed. Gently pull forward on the plastic trim and route the bundle of cables over the cargo anchor and up to the other bundle of electronic cables. After this, you can replace the piece of insulation you removed in the step above.
Now just apply some adhesive backed velcro to the MPM and the relay and place them in the positions as shown. If you place them here you will not have any issues with the electronics contacting the interior trunk trim. Also note that you just need to rotate the MPM behind the small accessory compartment (I keep a tire pressure gauge in mine). You do not need to disconnect the MPM from the wire harness to route the MPM behind the compartment.
At this point you can use a cable tie on the bundle of wires if you want to hold them together a little closer. However, I'd say this is optional as the cables will already be somewhat taught and should not move much.
This might also be a good time to clean out the spare tire wheel well. Also seems like a great new place to put a few bags of ice and a case of beer when tailgating.
Now just put everything else back together and turn on the car to confirm everything works!