Mike,
You are most welcome sir!
An add-on for mounting an extra switch/control for one (or more) of those specialty service vehicles is probably right on target. Perfect place for a switch for a big bubble rotating warning light or something similar.
Wonder if any fellow Bolters that may an example of one of those type "service" related trucks can shed some light???

Did you ever come up with more info on the "turn signal" part of your question???
Dan