The heater part is typically not grounded as far as I know. The heater will heat whether or not you wire the two current carrying wires to match original or reverse them because you don't know. The ground is probably important to get correct if it was used originally.

Reminds me of my electric water heater elements. They used to burn out every few years and would cook out sediments from the water because they got so hot. I rewired my heater so they worked in series ( not in parallel ), and now both heat at the same time but with reduced heat for each element. Works perfectly and now they haven't burnt out and no sediment buildup. There are just two of us and we've never had a problem with water not being hot enough. Also used the curly 'sandhog' type to spread the heat across a larger surface area. Also expect water heater to last longer because of less extreme heat cycles.

Don't know why water heater companies don't have that option during install. It does extend life of elements and does reduce sediment buildup.