Seems to be an older modern T/S incorporating trailer function. 18 wheeler/big truck thing. I kind of recommend changing that out, but if you want to keep it and are game to determine wiring, we can do it. You will need a combination of visual, voltage and comtinuity tests. Do you have a multimeter and an ice pick tester?
Post some more good pics of the Grote and it's wiring. Top, bottom, sides and some wires.
Do you understand my posts? and how rear lights are wired? Did I say anything wrong? Do you have questions? The problem in the front is easy. We will get to that later.
Seems like a little corrosion at grounds, a little miswire and old wire and terminations. Maybe a switch. All typical, lotta work sometimes.
Sorting the T/S wiring destinations will solve 90%.
