Lights and wiring are pretty simple...
...as long as you have a test lead. (Lamp or meter, doesn't really matter.)
To make sure we're talking about the same thing...
- Tail lamps also known as S/T/T lamps (Stop/Tail/Turn)
- Marker lamps
Front turn signals and tail lamps have three leads:
- Common Ground (12V-)
- Tail (Dim) (12V+)
- Stop/Turn (Bright) (12V+)
Markers have two leads:
- Ground (12V-)
- Power (12V+)
Your light switch probably has at least 4-5 connectors
- Headlamp (12V+) - Typically goes to the bright/dim switch.
- Tail lamp (12V+)
- Park (Marker) Lamp (12V+) (Some switches simply tie the park/tail off the same lead.)
- Overhead/Courtesy Lamp (Variable Rheostat+)
- Power In (Feed) (12V+)
With 12+ to the switch, turn it on and see what leads are hot as you cycle through the various positions. Some headlamp switches have "dedicated" feeds for the headlamp circuit.
With the switch "hot" and in the park position, you ought to be able to find the correct leads at the front/back for your tail lamps and markers. Both LEDs and incandescent bulbs are only ON/OFF, so as long as you have the correct lead from the lamp tied to the harness, you'll be golden.
It sounds like you have your stop/tail leads crossed if you're getting a bright tail lamp with minimal change for brake/turn. Probably same deal in front on turn/marker. Get the rear S/T/T lamps sorted properly THEN connect the markers.
Good luck.