question about a 86 nova/corolla

my brother has a 1986 nova with the 4 cylinder toyota engine. he has problems with the tail lights shorting out. i removed the dimmer switch from inside the car and that has corrected the problem with the tail lights but now there are no lights in the dashboard making it impossible to see the speedometer at night. i ordered a new dimmer switch thinking the short was in the switch its self, but after installing the new switch and turning on the lights it imediately blew the fuse again. any idea what it is that is causing this problem???