5toyta: on the 1978 type it is the 4 way flasher that causes the turn signal not too work if no turn signal. pull 4way 3 or 4 times then turn signal will usually work. According to Haines manual 1978 truck the flasher is a large square block mounted up under the dashboard. it does not look like the after market one. recommended u get a Haines truck manual 1978.