or borrow one. don't cheap out on one though. it all depends what you want to know. do you want to know what voltage your battery is holding or how much voltage is your amp seeing, which is more important to you? I would personally say do the amp so you can see what voltage it's seeing so that you can tell if it's close to going into protect.
I would do a run of remote wire to the HU personally, but you might be able to connect it to remote on the amp. I don't actually have a voltmeter in my car but I've looked around at what my friends have done with theirs.