making a power plug for xm

If there was a cigarette lighter type of plug for the unit, there is a chance that it is a 5 volt unit.  If it does not have a 3 or 4 pin plug on the unit, do not try the below procedure.

If there are 3 or 4 pins on the connector, I can tell you how to figure it out.  The ground pin will be connected directly to the chassis of the box.  Once you find that connection, put your meter on the diode test function.  With the black lead on the ground connection, touch the red lead to each of the other connections, let it sit there for a few seconds, notice the reading and write it down.  After you have written down the number for each of the pins, now place the red lead on the ground connection.  Touch the black lead to each of the connections and write the results down.  The constant wire is the one that read a really high or open reading with the leads one way, and read between (.)400 to (.)600 with the leads the other way.  Connect those to their appropriate connection.  If there are 4 pins on the plug, one of them should have given some kind of reading and the other one would probably not have given any reading at all.   The one that did give a reading is your switched power source. 

There are 4 pins but I think only three work. I took the board out and checked the resistance between the ground of the antenna and ground of the audio-out RCA's and each pin and I think I found the groun (read something like .001 - .01 ohms). I also checked the same pin to the negative side of the two capacitors and had the same resistance reading. Likewise, I did the same for the positive side of the capacitors to each pin and I think I found the positive pin. I think I was able to pinpoint the remaining pin, which I am thinking is a switched 12v. If I had a picture, I could double check my results.

I'm not familiar with the "diode test function". I use a Blue Point DMM...

It should be the only selection with a picture or diagram on it.  Following is the symbol for a diode, it should look very much like this

Excellent. I found the main 12v and ground then! The last post for sure helped tremendously. So I suppose the remaining pin that actually seems to be active (there's one that seems to be connected to nothing and one that seems to be connected to something) must be the 12v switch? is there a good way to test that other than apply 12v?

I found the diode test selection. if it were a snake, it would have bit me.