How to wire under the car?

[01ExSport]

just drill 2 holes and use 2 grommets, there's no pretty way to make a good seal putting 2 runs through one unless you use something like silicone to fill the gap between the wires and grommet.

[bassface]

I think using wire sheathing also wouldnt be a bad idea for a full undercar run. It may prevent any possible abrasions.

[01ExSport]

I always use flex conduit. I get non-split loom so nothing will get in it with the wire, but there's always the debate about holes for water and debris to fall out of.