'76 C10...front right parking light works, but won't blink

[Tantrix]

As the title says, I have a stubborn front right blinker that refuses to work. I replaced the bulb to double check that, and same behavior. Here's what it does/doesn't do:

1) Turn parking lights on, all 4 work perfectly
2) Turn left turn signal on, both front and back light blink
3) Turn right turn signal on, only back light blinks...front right one doesn't light up at all.


Is it a fault somewhere in my wiring going to that front right parking light/blinker? If so, why will it work fine when parking lights are on but only the back right will blink when turn signal is on? Any of you ever run into this?

Thanks in advance.