Does anyone know how to test the air compressor for the rear suspension to find out if the compressor itself is bad or if the height control box located by the rear shocks is the problem.

 

Dealer has tester. This will be general no clue what year or what is wrong.With key in run.You could disconnect link from control arm and move as high as possible, compressor should come on.Once compressor is on lowering arm should turn off compressor,it lower to lowest point may hear vent soleniod release air from compressor or notice air bags on shocks deflating. Need more info to give more info.