I recently stripped down and glitterized an Epiphone Special that came equipped with dual P90 pickups. After not really finding any decent wiring diagrams for this particular set up on Google, I decided I should just draft one up in DIYLC.
This is the standard wiring as I saw it when I removed the backplate; the only change I made when wiring it all back up was to replace the .022uf brown chicklet capacitor with a .022uf yellow polyester film capacitor. The change is actually very noticeable to my ear, it really opens up the sound.
The original soldering was pretty poorly done, I would suggest opening up your Epiphone and checking it out for yourself. Notice how poorly the pickup selector switch is soldered (I had already started to shield the cavity when I decided on the cap upgrade).
Cleaning up the poor soldering, upgrading the capacitor, and shielding the electronics cavity have really improved this guitar. Plus, now it’s glittery!