I am assuming that they aren't pulling many amps. Although I'm not 100% on that since I have never hooked them up. As long as they are fused at 15A or less you don't need a relay. You can use a fuse tap to get power, make sure that you use the hot side of the fuse though(fuse out you get a 12v reading on your voltmeter). Reason being is that if you use the fused side then more than likely when that accs. is used the fuse will blow.
Now you could also use a relay if you wanted to be on the safe side. Then you would just be using the fuse tap for a trigger for the relay.
Always be safe and fuse any accs. I've seen too many people not fuse things and blow their fusible links. Although I'm not sure if we have those.