2005 IS300 stereo head unit replaced GPS not working

[Stumpyau]

Just replaced factory head unit with an aftermarket one. Bypassed amp but now the factory GPS will power on with Lexus and then go black. Screen tilt works but no maps. If I plug back in the factory head unit GPS works again.

Any help please?

[Millerman]

The headunit obviously is used to keep it on 

if you have a test light or dmm test what your getting at gps with radio plugged in and then compare with radio not plugged in