[spdbrake]

I ran into much the same issue on my BDR. Rear turn and brakes worked fine with the LEDs. Added the front turn LEDs and then no front or rear flashing. Tried another flasher unit like Unique suggest and no change.
Added resistors to the rear turn signal circuits and all worked fine.

Then I ran into the same issue with the headlights on. The early BDRs used the Hella headlight with the built in parking lamps.

I switched my front turn bulbs back to incandescent and planned to revisit the project once I switched to Lucas style tribar headlamps. Which I switched out about a year ago. I'm glad you brought this up as I'd forgotten to finish, lol