Ade
It sounds like you have disturbed and earth connection or one of your capacitors ( suppressor)has gone bad.
Check all earth studs generally they have more than 1 wire attached to them. If you look at the cct diagram that was sent to you, you will see the signal = GRD going to a group of wires this is your earth point make sure this is not loose.
Capacitors are fitted to Alternators, Wiper motors, Coils, basically anything that sparks, You may have a faulty spark plug wire.
Have you used screen cable for your speakers, Have you redirected the loom closer to the ignition wires. If you think it is aspark related thing fit a capacitor 0.1uF 50Volt between the head set and earth ( chassis ).
If you have a multi-meter you should be able to trace this problem yourself using the ohms scale. I would only visit a specialist if 1/ I had a friend working there and 2/ if I couldn't find it after doing everything myself.
PLS check your mailbox