Thought I would post this...just because it took me nearly 2 months to find it.
I bought my Jeep a few years back, its a 99 Wrangler, it had 78,000 on it when I bought it.
I noticed on the drive home, when I turned on the left turn signal..all four lights blinked, as well as bot indicators on the dash..the radio display, and the dash lights.
I bought it as is, with no warranty... and I loved the Jeep so there is no way I was going to make them fix it or take it back.
Anyway..I called a Jeep tech, he told me it was probably the "three way switch" where the hazard light swtch is..so I ran down to a pull and save junkyard and got me one...took it all apart, put it in..and..same problem.
After two months of hanging my hand out the window everytime I wanted to make a left turn, I went to work one morning , I get there when it is still dark outside..guy I work with told me I didn't have a left tail light..so when I got off work I stopped by pep boys and grabbed a bulb.. when I pulled the old bulb out. I compared it to the new one I bought just to make sure I bought the right bulb..and it was different.. so I ran back up to pep boys and told the guy he gave me the wrong number from the computer..he double checked it and nope..he was right.
The bulb I pulled out was a single element bulb..it has one connecter in the center on the bottom...but the socket has two connecters that touch the bottom of the two bulb that is SUPPOSED to be in it..that single connecter on the bottom of the WRONG bulb..was touching both connectors in the socket and back flowing DC through the whole light system and lighting up everthing when the DC ran to that bulb through the blinker wire.
Funny thing is..the WRONG bulb actually had to be forced into the socket because the little "push and twist pins" on the bulb that hold it in the socket..were slightly off..so whoever put that bulb in there actually had to push so hard that it kind of bent the socket out in order to allow the bulb to fit.
Anyway... just though I would share that in case anyone else has all four signals blinking when they turn on a turn signal, I would have NEVER thought to check and make sure some moron didn't force the wrong bulb in the socket!!!!! I just got lucky and had the bulb go bad pretty quick after I bought the Jeep.