I just picked up a used Terk Commander XM radio for my 94 E320. It has 3 wires:
1. yellow for 12v; can I get 12v at the car's electric antenna? Is there a better source?
2. Black for ground.
3. Red to Accessory. But it seems that my ignition switch has 3 positions: off, on and start; no acc. In this case, the instructions advise to connect the red wire to a terminal connected to the ignition switch "on/off" position. Where is this?

regards,
Mark

the lead to the cigar lighter is the best location in my opinion, be sure to use an inline fuse for your unit