1998 Grand Cherokee heater fan does not work If I hit the dash board it turns on?

Fuse, blower motor resistor pack, blower motor, and blower motor switch, in that order.

Failure of the blower motor connector on this model is common. Check www.wjjeeps.com for pictures and an explanation of the problem. Sometimes you can get temporary operation by jiggling the wiring connector.

You also need to check for the source of the problem. It is also common for the re-circ door to break, blocking air into the fan and causing it to overheat, stressing the wiring. Check the HeaterTreater re-circ listing for diagnostic information and pictures.