Check for;
a shorted or bad bulb
bad ground in socket
A mis-wiring or short circuit somewhere between the controls, fusebox and lights.
Bad ground.
Replace the turn signal flasher relay.
If the right one works,it might be a bad bulb or a bad ground.
That sounds like a grounding problem, probably near the right side lights. It might be a rusty or disconnected ground wire.
Sounds like you need a new flasher.
1989 silverado left turn signal set applying brakes causes the left turn signal to dim and the two right rear tail light bulbs to alternate brightly? Further problem occurs to same truck - in Park and brakes not applied -- if head lights are on turn signal flash slows right down almost to a stop and becomes very dim
POSSIBLY A BADGROUND
You have to replace front and rear bulbs.
When it flashes fast it means you have a burned out bulb omn that side. Check all you lights and you'll find the one that's burned out.
It has been my experience that a rapid turn signal indicates there is a burnt-out signal light bulb (either front or rear) on the side of the rapid blinking signal. If the right signal is blinking rapidly, the problem is in the signal lights for the right side (front or rear) of the vehicle. Turn the rapid signal on and step out of the vehicle to find which bulb is not lighting.
I would replace the turn signal switch.