TAC module problems

So I've mounted my tac module on the drivers side firewall on my 95 for the dbw. Problem I have is it doesn't work when it's mounted, if I remove it and try it on another truck it works fine. Ive tried three of the stupid things and the only way I can get one to work is by having it just laying in the engine bay.

Any ideas ?

sounds like bad connector or some loose connection on the harness that makes a better connection when you lay it down. id search for bad connections/wires

Thanks I'll investigate closer.