signal light leds

having a problem with installing some leds in the signal lights. Problem is when teh leds are installed they blink quickly, low resistance making car think that the bullb is burnt out. Have tried a resisor in parallel which should fix prob but it is not. Everywhere I look it says this is a prob but can't find a solution explained. Any help would be appreciated.

guess I should add it is a 1994 mustang

When I was looking at buying some leds for my truck I was told if I had any problems with them to get a manually adjustable flasher from the parts store.

check out the website www.waytek.com they sell flashers for LED lights.