Have a question, neg when running.

Hi, I was wondering how you can find a wire that is negative when the engine is running. I have a 96 ford Ranger pickup, and have tried to replace a old commando 5000 with a commando 5005. I am having problems with the LED comoing on when i step on the brake when i am driving and think it could be because i do not have this wire hooked up. I am having problems getting it to go into program mode also. Everything else works fine, remote start, and alarm triggers. Thanks, hope to hear from someone, have a great day.

Hi first why would u need a ground when running. second what wire dont u think u have hooked up. Need more info

Hi again, well I went through the 5005 manual again, and it seems that the wire that i thought was supose to go to ground when running was actually a ground when running output from alarm, probably for another channel. I hooked up another wire that got its power through the 5000 the first time from the ignition, now everything works except my radar sensor. Thanks for your help, have a great day.