Power pack may not necessarily be bad... Indeed the power pack is AFTER the dimmer rehostat, so if you are getting 12 volts at the orange wire at the power pack, but not getting the nominal high volts AC on white wire, I assume that you are making measurements with power pack connectors mated ?? Since this problem is intermittent, you can only find the problem when the EL is NOT working. You would only see the high voltage IF the EL is illuminated and working. When not working, you will need to use clip leads or other means to hook up the power pack and INTERRUPT the white wire yet still have ground and 12 volts applied to the orange wire. Measure for hi volts on power packs white wire going no where, connected to nothing. If power pack is working, you should hear a a low volume high frequency "squeal" coming from the power pack. If you have the hi volts AC with this test, when you connect the white wire to the feed the EL devices and loose the hi volts, then you have a dead short somewhere among the Electroluminescent items. If there is a short anywhere on the hi volts white wires path or anything EL related, the power pack shuts itself down.
EL items are : Gauge background buckets, gauge needles, radio pointer, clock face and hands, shifter indicator. Only way to find out which item is the problem is to disconnect items one at at a time till you find whats defective. This short cant necessarily be found with an ohm meter as it may only be shorting with hi voltage applied.