if the handbrake light isn't working ,personally i would check the switch and the wiring on the park brake relay valve...
whoever told you the switch is controlled by the air drier/ apu is getting confused...on 75 85 95 and 105 .if the handbrake doesn't release and the light stays on then the airdrier /apu is generally faulty ..but from your fault description ,it sounds like switck or wiring
norb
September 2012