If you get a click when you take your foot off the brake pedal, then I would suspect that your amp is not properly suppressed. Releasing an inductive load in a car causes a power line spike. Does it have a power line filter?

I have also found on a number of occasions with radios that if the unit is poorly earthed, the unit becomes sensitive to power line (and any other) noise (alternator, switches, wiper motors, etc.). This is due to a high-impedance return path from the unit to the chassis.

I'd check the earthing of the empeg and the amps, which should be earthed with individual lines to one point only; the line wire thickness should be sufficient to handle 50% over peak load from the device (I'm sure tanstaafl will be able to advise better on wire thickness here). Finally, both the amp and the empeg should have their own power line supressors of the correct continuous current rating; if you (for example) use just the empeg's choke for both amp and empeg, then the normal current drawn by the units will cause the core to saturate and reduce the suppression of transients.

One of the few remaining Mk1 owners... #00015