Why do you need a rev counter in an automatic ?
Anyway -- either get one from a 411 / later V8
Or this may be useful
Positive to Negative Ground Conversion
Electronic Tachometer Wiring Reversal.
First, read the front of the tachometer. If it IS marked "Negative Earth" it probably IS negative earth already. But then, you probably wouldn't be swapping your car's electrical system, and wouldn't be reading this. If it is marked "Positive Earth", and there are no other labels on the back housing (notice this after removal of the tachometer from the instrument panel) indicating that it has been changed over, follow these instructions.
Remove the tachometer from the instrument panel. Look at it carefully. Draw a picture of the orientation of the connections for wiring, attachment screws, note markings etc. and... most importantly... note and carefully draw a picture of the orientation of the loop of white wire running through the plastic holder. This will be reversed later in the instructions.
Now turn the tachometer over, glass face down, and locate on the sides of the tachometer, the bent tab edges of the chrome (or black) bezel. Use a very fine-blade screwdriver to wedge between the tachometer body and the bezel tabs. Bend them out ever-so-slightly one-at-a-time, turning and rotating the tachometer, regularly trying to rotate the bezel free from the tachometer body. Go around and around. The very least amount of bending of these tabs is best. When the bezel finally rotates free, turn it until it disengages from the metal body, remove and set aside. Carefully... very carefully, pry off/out the metal shroud and glass from the tachometer body if it did not come off with the bezel. Be really REALLY careful with this glass and shroud, as it is easy to damage either or both. Also, be careful to take care of whatever seals you find here, as replacements are not available...
Set the bezel, glass, shroud and seals aside to clean carefully before later replacement.
Once the glass has been removed, be very careful not to disturb the needle, or leave fingerprints on the face of the tachometer. Turn the tachometer face down, looking at the backside. Refer to the drawing you made earlier. Locate the 2 slotted screws holding the mechanicals of the tachometer inside the outer housing. Unscrew them carefully, slowly, until they are almost free. Pick up the tachometer in one "soft" hand covered with a tissue to prevent fingerprinting-about-to-happen, and complete removing the screws. The tachometer "guts" will fall out into your tissue-covered hand, hopefully, and not onto the floor.
Look carefully at the internal wiring connections. Look for the small 1/2 inch long brown, striped, diode. It is connected to the metal tab that provided electrical power into the tachometer. Nearby, look for the small green wire, soldered to a metal lug that connects to the metal internal frame of the tachometer, providing a ground. Unsolder these two connections, reverse the connections and solder in place. Do not use any more solder if you can avoid it. Just heat, remove, swap/replace, let cool. Now the small brown diode connects to the ground lug, and the thin green wire connects to the "power in" lug. This is now correctly wired for "Negative Earth", as is the picture below.
Take the time to carefully dust off the tachometer face (a small artist's paintbrush is ideal for this). Replace the "tachometer guts" into the metal housing, taking care that any rubber shrouds for the marker lights are in position as the "tachometer guts" are lowered into position. Align the screw holes, and replace the screws. Clean the glass, shroud and bezel, and replace. CAREFULLY re-bend the bezel tabs to grip the tachometer body.
Look carefully at the loop of wire that runs through a plastic holder on the back of the tachometer. This must be reversed, so that electricity "flows" the other direction. You can... follow the wires until you find connections and reverse them at that point. Or... you can carefully un-thread the white wire from the plastic loop holder, and re-thread it correctly, reversed "flow".
Attach a small label to the backside of the tachometer housing stating the "modification to negative earth", Date it and initial it.
Replace the tachometer into the instrument panel, re-attaching all wiring.