My DW and I use walkie talkies and they work great. I have a cheapie set I just leave in my truck year-round and they go thru 6 AAA batteries every two years or so. We tried using our cell phones (on speaker) but we have different carriers and don't always get a good signal so they are usless at certain CGs. For night backing without walkie talkies, you could try having your spotter use a flash light or chem-light and make up some signals. Use the light to point the driver in a direction...cover up the light or hold it up high to indicate "stop".
