iv got a little bit of a misfire at low rpm on my 99 sts,ive already changed the ignition wires and plugs with new ac delco from rock auto which has made it considerably better.i am now considering changing the ignition coils, rock auto has 4 types ranging in price from$20.00 up to $48.00,anybody know what the difference is?

Looks like the top and bottom coils (TruTech and ACDelco)(least and most expensive) are the same unit.

Your call as to what looks closest to your OEM because there seem to be a couple of different part numbers.

It makes a very big difference which plug is connected to which coil tower. Be sure you don't cross wires or coil towers.

If you didn't keep everything exactly as stock when you did the plugs/wires, you may need a Factory Service Manual or a diagram from a '99 owner in here to get it right. There have been quite a few posts concerning missing and backfiring in a "new" 4-coil waste spark system. Don't forget that 1 coil fires 2 plugs at the same time, one piston near TDC on the power stroke and the 180 degree opposite one near TDC on the exhaust (no gas to burn) stroke.

The factory coil packs should be marked with the plug numbers for each terminal. If you buy replacements you should transfer the numbers as you replace the coil packs for future reference.