It doesn't make sense for the fronts to be on solid while the rears flash.
Are you saying they are dark then, say you activate the left signal, the rear left flashes but the front left comes on and stays on not flashing?
The original system is a bit complicated, not certain for a 77 550 but they should be like this:
With key on, headlight on...
no signaling - rear signals off, front signals lit on dim filaments (marker lights)
Signal left - front left dim filament off & bright filament flashing, rear left flashing bright (only one filament, bright)
Signal right - same but other side.
The flashing signals are powered from a grey wire that goes from the switchpod to one pin on the flasher unit. The other pin of the flasher goes to black +12V "key on" power. Both signal filaments (front and rear) connect to the pod signal switch, activating a signal connects that flasher power wire to the signals. There is no way to have the rear flashing and the front come on solid unless you have a wiring mistake.
The left signal should be an orange wire, right light blue. The marker wires are the same colours but have (had?) a little white plastic thing (little donut) near the bullet connector. If you test one with a 9V battery you can identify which wire is positive to go to that orange or light blue wire. Wire colours from things like that from China are unpredictable and extremely unlikely to match SOHC4 colours.
Right now I can't imagine a wiring connection that would do this with reverse wiring the LEDs and getting back fed power but it may be possible?
