I had a similar problem when I hooked up heated gloves to the bike.
First, remove the leads for the new gear.
Second, un-hook the battery, and leave it off for about 10 minutes to make sure your memory clears and resets.
third, hook up battery, see if you are back to normal with the tach. I'm guessing you will be.
Fourth, assuming your are back to normal; wire in the positive DIRECT to the battery WITH an in-line fuse.
Fifth, wire in the negative DIRECT to the battery.
See if that works for you.
Now then, if you don't have a switch, or want it to come on and off with the bike or a particular switch on the bike, wire up a relay to control the power, but keep your direct connections to the battery. That should have you in business.
If the tach doesn't come back to life at step three ... then you are outside of my expertise.
-GL