[dana44]

It almost sounds like some of the wires back there are in the wrong locations.If not mistaken, the high beam and the turn signals are independent and on wires, not the circuit board. Has any of this been taken apart in the past? Wires in the wrong locations is an idication that the high beams flash when the turn signal is on, so check your wiring diagram first, trace a couple wires for the signals and high beam, make sure they are the right color and right location on the panel. You can also check to see if there are wires that have melted together, which does happen, crossing power from one to the other, but that doesn't sound like your issue.

 

[valiant67]

A very common area for problems is the bulkhead wiring connector at the firewall. The seal isn't very good and there can be lots of corrosion. Besides the possible problems Dana has mentioned, corrosion at the connector can do strange things, often interrupting grounds and causing one circuit to find ground through another circuit (which can account for the bulbs lighting up in the wrong place).