You can hook the Solargizer up to the batteries directly if thats more convenient. I know this is known by most people but for those confused, The 4 battery system works as follows. The first 2 of the 4 batteries are wired in series to make one 24V "battery" made up of 2 of the 12v batteries. Then the next 2 12v batteries are also wired together in series making another 24v battery source. Now you have 2 separate 24v sources of power. These in turn are wired in parallel by connecting the two left over neg terminals together and the leftover pos together to make one even larger 24v source. This is the source that powers the truck's 24v system. That is why you can use on Solargizer to essentially charge all 4 batteries. You can identify the 24v source as the inter vehicle jumper plug is wired to them as well as the starter. Series wiring in this case with the 2 12v batteries making one 24v is done by just taking the pos terminal of the first 12v battery and connecting it to the neg terminal of the second battery, and then using the one leftover pos and the one leftover neg terminal as a source of 24v, neg terminal going to ground, pos going to 24v load.