Many thanks to everyone for their kind comments; they are very much appreciated, not so much from the point of view of the praise as from knowing that you've given someone else some pleasure/enjoyment as well as yourself, if you follow me.
Unfortunately, what I really wanted to customize were the two 1900 (Belle Époque) vehicles. Now they are so valuable, that I daren't touch them.
Hopefully, Zirndorf will reissue them as they did with the bi-plane. These two vehicles would really lend themselves well to historical fire fighting equipment as well as to late Victorian military vehicles.
I'm sure, Richard, that there are some customs of these as turn of the century fire engines on playmo-portal by flori. I'll see if I can find them for you.
I like the jump seats in the back, thats a very good idea. 
It gives me lots of ideas for my own custom unit (that should be done soon). I'm looking forward to seeing this unit in action.
As I'm sure you've guessed, Ben, the seats are cut out from the cab interior of the ambulance. It was easier to do than I expected. I then mounted them on some plastic (ABS) angle pieces to get the correct height and glued them by the back to the dividing partition. They are just too near the floor to make use of the space between the supports for under-seat storage, but having them higher ruins the "balance" of the setup.
the customized warning lights with some "real" lenses
These are flat-backed "jewels" with faceted (angled) face used by craft workers for jewellery. I'll scan some examples as they come in different sizes and colours and post a pic of them.
Very nice details, e.g. the "structured floor"
I too like the extra seating in the back and the textured flooring used in that part of the vehicle. 
The textured flooring is made by a firm called Wills for HO/OO railways. It's really designed for making walkways on gantries or weighbridges but works equally well at Playmobil scale for flooring or for rooftop storage areas. It comes in a medium grey, but I had to paint the floor because it got marked by some polystyrene cement.
Once again, many thanks for taking the time to look at my custom and comment on it. And I hope to have some action shots once I can get something set up outside again.
