Freaky fuse 21 problem

[URNVS]

My 318i keeps blowing fuse #21 (instantly). I traced it to a 4 pin plug under the drivers side dash, and when I unplug that plug the fuse no longer blows. The funny thing is that the plug appears to go to the radio but I have the same problem even if I disconnect the radio. To make matters even more confusing if I check continuity to ground it is present at the fuse with the under dash plug connected, not present at the fuse when the under dash plug is disconnected, not present on any of the 4 pins on the under dash plug, but present on all 4 pins of the socket that it plugs into (?!?).

Im really lost and would appreciate any input, and if anyone is willing to share the under-dash wiring diagrams for my car please let me know!

[Starmavin]

First off there are a few things I need to know: When did this first happen? What are the wire colors? Is it the factory raido? Do you have or does the power antenna work?

[URNVS]

It is the factory radio, it has a power antenna dont know if it works yet, I just bought the car and the problem came with it, the wire colors on the 4 pin plug are red/green stripe (that color combo is on fuse 21), brown, grey/red stripes, grey/pink (?) stripes.

Thanks!

[Starmavin]

There are a few things I want you to check, first off is the small pocket in the glove box to the left, you'll see a pice of foam or two pins. This spot is for the factory flashlight, check and see if a penny didn't get dropped in there. Next check all the interior lamps, glove box lamp and even the one in the trunk for a short. Last is that your service interval board in the speedometer is shot. But check everything else first.

[URNVS]

Interior lights & trunk light work with that 4 pin plug disconnected. There was a foam piece in the glove box, it has +12v on the pins if I remove it. My speedo works but the RPM meter is out. Where is the glove box light located?

[Starmavin]

The glove box lamp in located in the upper plastic housing that covers your ecu in the glove box. If your RPM guage is out, then there is a very good chance that the SI board could be the cause. Do you have any SI LED lamps light up when you start your car?

[URNVS]

SI LED's are the ones at the lower right of the cluster? If so yellow and red. I actually pulled the cluster just now (what a spectacular pain in the ass, hope the bulbs never burn out), disconnected all the cables then re-connected them one by one. None of them blew fuse 21 after re-assembly...now it doesnt trip anymore?!? RPM gauge and OBC still however dont work. Where is the RPM sending unit located?

[URNVS]

I just pulled out the SI board and checket the batteries (lithium), both are at 2.9v. I do have access to another cluster so I suppose I can try swapping them tomorrow.

[Starmavin]

Look at the wires for bare cores, if you found no real cause that is. A wire short that broke it's short to ground when you moved the wires my be your problem....