Underneath the fuse panel is where I like to make my battery connections. Since I do not like to have more than a couple cables going to each post on the battery itself, I use a power post as a junction block for extra connections, like the winch.
I may or may not buss these two post together. When I get to that point, I'll decide.
Fitting the ECM connectors through the firewall is a major challenge. There's really only one spot it can be done. On FJ62's, this hole is already there, but on FJ60's, it has to be punched out. I use my grandfather's electrical box knockout tools for this job, since they make a nice, round hole with only minor cleanup needed.
The hole is huge, and almost doesn't fit, but it has to be large or the ECM connectors will not fit through.
In the past, I have used OEM Toyota firewall grommets for the sealing of the hole, but the source of good ones is drying up and they tend to be on the small side. I still don't know what I am going to do to seal this hole, but I have a few aftermarket grommets on order and I'll try each one for fit. I wish someone sold OEM style firewall boots for large diameter holes. The majority of the grommets out there look like they belong in a billet '32 Ford roadster. That's not the look I'm going for...