Regarding the mounting of a light bar on the roof of (whatever): ALWAYS try to mount the legs of the bar on the rain gutters. Doing it any other way will cause problems when you take the apparatus out of service. It's worth it to obtain a light bar with a wide enough span to reach gutter-to-gutter.
And, if you check for "used fire apparatus for sale" on your search engine, you'll find an occasional 4X4 ambulance for sale, including some medium-duty ones.
I'm a member of an emergency vehicle upfitter's group, and this question comes up periodically.
Generally, most police vehicles have gutter mounted light bars. These vehicles are in daily use, and have a relatively short life in a municipal fleet, so resale can be a concern. Increasingly, police vehicles are leased, with prohibitions against drilling any holes in the body, so a gutter mounted light bar (or an interior bar) becomes the only option. Routing the power cable can become challenging, though.
Fire apparatus has a much longer life with a municipality - they are fairly low mileage, and are parked indoors, so deterioration is much slower. I have seen long light bars bolted to the apparatus roof and I have seen a pair of shorter light bars end to end bolted to the roof, but I don't think I have ever seen a gutter clamped light bar.
New ambulances tend to use the manufacturer's body-mounted emergency lighting, though body bracket mounted lightbars are occasionally seen, as are roof mounted light bars. I have never seen a gutter mounted light bar on an ambulance, even on the smaller ambulances. Ambulances operated by fire departments seem to favor lightbars over body lights.
