Well I just did this mod in my Grand Prix today. Its significant if the slow blinking of mechanical flasher bother you, or if you just want to modernize your car a bit.
You only need 4 things... head to the junkyard and find the flasher relay along with the wiring harness out of a U-body van** (Venture, Trans Sport, Montana, Silhouette), two male crimp connectors, and one eyelet connector.
First, take your flasher relay and crimp (or solder if you have connectors that accept it) the two male end connectors to the blue and purple wire. Do the same with the black wire, only with the eyelet.
Second, locate the mechanical flasher in your car (near the steering column almost at the fire wall on 88-93 w's) and remove it. Then take the harness and plug in the male end connectors, the wires are the same color so just match them up.
Third, find a suitable ground to bolt the black wire up to. I used a small ground just above the ALDL connector.
Last but not least, find somewhere to tuck the flasher (the original location works if you have enough length on your ground wire)
Enjoy your faster blinking, more modern sounding turn signals!
**The flasher relay of the u-body vans is located to the left (sitting in the drivers seat) of the steering column. Remove the knee panel and lower carpeted panel and look up. You'll see a block with blue connectors in it, look just past that and you'll see the flasher. Also, not ever U-body has the flasher in the same spot, so if you don't see it there, you're not going nuts. Unfortunately, if its not in that location, I don't know where else they like to hide them, so I'd just move on to another van.
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