Why does my engine go dead when I put it in reverse ?

Suggest wiring harness wiggle test with engine on to see if engine stalls, may need to remove engine cover inside van .. When shifted into reverse, engine movement may be causing wire harness to shift and creating stall but does not do it when shifted into drive as engine moves in the opposite direction... I can't think of any-thing else that would cause an engine stall when shifting into 'reverse' only!