intermittent park light function sounds like a ground problem .... I've never seen a burned out bulb affect the operation of the rest, but there may be modern solid state flashers that would do that due to load change .... with the rear light on steady I'd suspect the flasher itself or a wiring problem, the signals serve to interrupt the brake light
see if you can hear the flasher clicking when either side is selected, it may only be getting power in one position. which I'd think would mean a bad switch .... I know signal malfunctions can be frustrating!!
Bill
