URGENT!!!! No heat, no fan, no display

[Aislinn]

91 Chev Silverado 4X4....no heat! Originally, the unit would cut in and out- the display lites would go out completely when it quit. Then it would come back on for a while. Then it quit altogether.
We removed the panel, and found burnt electrodes, so we purchased a new unit $260.00 !!!!. Installed it this a.m.- SAME PROBLEM!!!! The unit came on OK as soon as it was connected. On adjusting the fan speed, it quit. And it hasn't come on again. Obviously there is another cause, but what?

Can anyone offer any suggestions? Can't drive without heat/fan in this weather.

Thanx in advance for ANY help.