Turn signal stays on when flashing - 2005 Mercury Mountaineer

Look for a bad ground on the circuits. If you dont have a good ground the circuit will back feed looking for a ground. Try it.

Hows your blinker fluid level?............. sorry old joke...check your wiring for groundouts or shorts then check your flash can (large silver round fuse) !

lol sounds like my old truck most likely crossed wires

My first guess would be a ground problem. The bulbs are mounted in plastic sockets and grounded to the car by a black wire. If one socket is not grounded it will try to ground thru a different socket.
I would check for voltage and ground on each socket in the back.

FLASHER device go to bad, replace it in order to solve.

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97 chevy blazer left turn signal makes the dash lights flash then when i turn parking lights on left signal stays on constent when engaged

change your turn signal flasher.

Check the the bulbs at the front and rear of the vehicle work. If one is not lit up, replace that one. On some of the earlier model cars the blinkers would stay solid and not flash when one or both bulbs were blown. Modern cars have a rapid flash when one or both are blown

its your blinker box.. all major lights run through there

broken ground wire