stupid "radio tach" !!

How does one get rid of annoying radio tach???? the noise through the speakers that mimics the engine rpm? I have replaced the main ground wire, engine to firewall ground and added a main grounf to the framerail on the drivers side, new cap, rotor, ngk resistor type plugs, and new ngk wires, any help or suggestions are apreciated!!

more firewall insulation, or thicker insulated plug wires

install a ground loop eliminator on the neg side of radio, can be found in most stereo shops and have seen them in kragen a nd autozone $15-$25. A must have if amps and other stereo equipment are used.

just keep your amp ground under 9" long.good thick ground wire good sized screw/bolt sand area were you plan to atatch ground to meatle

What year? Many Renix (87-90) engine computers put out interference that is very hard to eliminate.

its an 89, my xm will outpower the radio tach, but just normal radio or my mp3 with fm modulator and I get the noise

My experience is that you have to figure out if the interference is coming in through the antenna, is radiated into your electronics (cables or circuit boards) or if it's coming in through the power/ground cables.
If it's radiated interference, changing things around like the length of the grounds, shielding and whatnot can reduce or eliminate much of it.
If it's coming in through the power/ground cables a inline filter often helps. Though finding a filter of the proper frequency can sometimes be a trial and error type thing.
Sometimes running a power cable straight from the battery helps, they say the battery acts like a big condenser.