Same approach as for any car, I guess, and I've done it on a few in my time. Locate the mother-of-all-looms coming through for all the dashboard wiring where there is likely to be the most generous grommet real estate and very carefully push a wide-enough screwdriver through it. 
As an ex motor trimmer, (retired) this method is the recommended technique in the industry. Any holes through the firewall "could" technically change the insurance conditions and if the vehicle needs to have an engineering survey (pits inspection etc) it would not meet that countries specification.
Back to the large grommet, sometimes there is a large twist in the loom, on the other side of the grommet, so just be aware that you could be digging into the loom, so just beware of that.
Also, you can use a lubricant, such as silicone oil spray that will stop the rubber grommet from sticking on the screwdriver or tool. These grommets may actually have 2 rubber skins, like a firewall face and a loom tail to meet those firewall conditions. Applying lubricant will make the job a whole lot easier. (avoid something sticky like vasoline or that could damage the loom)
As a tradesman, I used a small tube, with a beveled end, which gave the advantage of feeding the wire through the tube (and the grommet) was a piece of cake, then unthread the wire from the tube. ( I had a set of tubes of different sizes, but I mostly used 1 hard steel one so it didn't bend ant tighten on the wire.
