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
The flashing lights on an ambulance are commonly referred to as emergency lights or warning lights. These lights are used to alert other drivers and pedestrians to the presence of the ambulance, indicating that it is responding to an emergency. The lights typically flash in a pattern to enhance visibility and signal the need for immediate right-of-way.
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.
I would replace the turn signal switch.