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If the switch is properly adjusted, the brake light wires must be shorted together. When you remove the connector from the switch do the brake light go off?
yes - it's the only way they go off. Even if the truck is turned off and the keys are out of the ignition the rear brakes lights remain on if they are connected to the switch.
I just took a look at the switch on my 87 Ranger & it does'nt look like theres any way to adjust it. If yours is the same, maybe that new switch is defective. I'd remove it & check it with an ohm meter.