all the wire get clustered together into two plugs that connect to the back of the gauge cluster, so make sure you know which ones EXACTLY to splice into before you do anything drastic.
I dont' know of DIY's on that.
Ask the people selling it if it comes with good instructions??
I know you probably already heard this. but you already have a tach so why do you want a big ugly one bolted on to your dash board ??