99 LHS, digital heat/air display does not work and blowing warm air.

Hello, it doesn't sound like a power concern, but more a control concern. It's difficult to say if the controls are faulty until testing is performed. There is a "self diagnostic" mode via the control unit, but if there is no functional display, this won't help. A compatible diagnostic tool that can read "body" codes from the body control module (BCM) is needed to further test. www.repairpal.com/repair-shops