I been interested in installing LED bulbs for my exterior lights for some time. I purchased 1157/1156 LED Bulbs with projector lens from Amazon. I thought I was going to need to install resistors but all I needed to do was change the flasher for an LED flasher. This really was a plug and play installation. I did read a recommendation to use the color of the LED to match the lens or the LED could wash out the color of the lens. So installed Red LED for the Brake & Turn Signal, Clear LED for the Reverse light and I have ordered Amber LED's for the front. The lights are very bright, I am extremely happy with the results. This is on my 89 S15 2WD 2DR 4.3L S15 Jimmy. By the way I now have about 494,000 miles on the body, frame. Still is my daily driver. Car runs great. I should hit 500,000 miles before the end of the year.
Yesterday I installed Front LEDs and Side Marker LED's. When I installed the front LED's the Turn Signals would not flash with the lights on. I installed LED resistors on the Front L/R turn signals. That corrected my flash issue. Bottom line I needed an LED flasher plus 2 LED resistors on the front.

 

I ran into a similar issue with my '98.

I didn't like how the LED Flasher relay worked (or lack there of), so I did the free trick with a donor stock flasher.
All the rear LED's work great.
I have HIDs headlights in the front, plug and play, with built in ballasts.

But I could not get the front side marker lights, or the front turn signals to work properly with LEDs in.
They work fine with regular bulbs in.

I will need to look at adding a few resistors it sounds...

 

I have LED bulbs in all my signal/parking lenses too. The only ones that don't work correctly are the little 194 bulbs in the front. They light up but do not flash. I just decided to live with it, unless someone knows a way to correct this?

 

Try reversing the bulb in the socket.