I assume this will work for any detector, but the wiring might be slightly different. I've now seen a V1 and an Escort 8500 mounted this way.
Here's a link to some pics that someone else took of a V1 install.
For the BEL 985 (and I would assume any other BEL detector), get the straight, as in not coiled, power adaptor cord. I recommend using the L shaped plug and cutting off the straight one. I left about 2 ft. of cord attached to the L shaped plug in case I ever wanted to move the detector.
If you look at the pictures of the V1 install above, you can see that there are 3 wires running to the mirror assembly. Splice the black w/ white stripe wire from the detector power cord into the blue wire for power. Splice the black into the black for ground.
I ran my wire out the front of the headliner. I'll post pics tomorrow.