You can have the tach dim with the dash lights. Also, I would use a voltmeter to figure out what wire comes from your coil to your ecu so that you can tap into the wire inside the car near the ECU rather than drill a hole in your firewall and hack into your coil wiring. Pull your dimmer switch out and hook both of your tach backlight leads to it. You'll have to use a meter to figure out for sure, but the way the dimmer works on B13's is that the dimmer changes the light level by resisting the ground from the lights. So hook your backlight ground wire to the dimmer wire that goes to ground and the backlight positive wire to the dimmer wire that has voltage present when the lights are on. Check around but I'm pretty sure that the wire from the ECU that goes to OEM tach's is abandoned in your dash somewhere, so use this to connect to your tach sensing wire.
Just to make myself not look like TOO big of an ass I didnt write up that how too just did a search for it. I dont have a tach installed it was just a cut and paste job so if you can have it dim I wouldnt know