1997 lx450

Guys! Help me please and I will p[ost the fix afterwards.
My 1997 LX450 (ie Toyota Landcrusier) has a new stereo Pioneer with dvd works great except I have to push my left knee on the drivers side door to keep it playing. The dvd and stereo is always lit, it is just that no sound comes out. sometimes if I whack my door handle it will play until I hit a bump, but then if I push my left leg against it it will work until I take the pressure off the door. Please help me fix this, I don't trust the installer anymore.

thanks!