Ok, I bought a 99 Grand Marquis less than a week ago. It has been converted over to a Marauder clone. When I drive at night for about the first 20 minutes, the lights flicker. The guy I bought it from says he didn't change the cluster in the dashboard, and the Grand Marquis cluster can't support the power from the marauder headlights, and that's why it flicker. So I took it to the shop and they said the previous owner didn't know what he was talking about and changed the module and said that was the problem. Ok, so I'm driving at night only this time it took a little longer, about 45 minutes into driving it starting flickering again. I don't feel like paying a shop double working for poor diagnostics so what do anyone that specializes in this type of work think?

Probably better to ask this in a “car” rather than “motorcycle” forum
Anyway, first check grounds. Often when older cars have collision or rust repairs the ground wires from frame to block, block to core support, etc, get lost. Unscrew, clean with steel wool, then replace the ground wire bolts. (a bit of elec. Grease as it goes back together is nice but not essential) This is a good start as it does not cost $ !
Also. Next time you drive at night for a while check to see if the headlight switch, ignition switch or high beam switch are getting hot. Corroded or worn connections at the switch can increase resistance and current demand.
One night, with the lights on, rev the engine. Do they brighten up, get dimmer at idle?
Probably the best answer is a relay, get a decent 12V relay and hook up the wires that now go from the headlight switch to a relay, run a heavy FUSED wire from the + battery terminal to the relay and from the relay to the headlights. It is likely that the brighter headlights draw more current than the stock units and need a bigger wire. This will make the lights brighter, as well.