Brake lights wont work? on 1990 Ford F-250

The truck is a 1969 F250, I changed the Brake Switch, Im getting power to Brake Switch, and tail lights. Could it be a bad flasher switch thats under the dash? I didnt replace it yet. Also the bulbs are good, I think?

Asked by for the 1990 Ford F-250
try your bulbs first. there are 2 elements in the bulbs . if you have power back there then you either have bad bulbs or a ground issue.

follow the black wires from the socket. they will be body mounted close by.

Thanks for the info, Do you know where the ground might be?