The basis for the conversion, Italeri's venerable Kfz. 15 Horch kit, makes a solid platform to build upon.
A period photo of the Phanomen Granit 25.
Pre-war, Phanomen built civilian ambulances and vehicles for the Deutsche Reichspost. Production of Sanitatskraftwagen (Sankra) for the Wehrmact began in 1936 and continued until the end of the war.
The Model 25 H (for Heer) was produced from 1936-41 and hundreds were delivered to the Wehrmacht. The exact number is not known but it was the most widely used ambulance in the German inventory. The Model 25 H was followed by upgraded Model 31 and 1500 vehicles.
Given the vehicle's wide-spread use during the war, I am surprised there are no 1/35th injection molded kits available of the Phanomen Granit series of ambulances from any major scale model manufacturer. 1/72nd and 1/87th scale scale kits are currently available from Attack and others but the only 1/35th offerings are vacuforms or resin (Azimut, Wespe) from decades ago that are impossible to find.

The conversion begins with the Italeri chassis and front end receiving a few changes to match the 25 H.
The interior (less steering wheel) is completed.
The back end is built up using 0.20 sheet styrene. The 25 H's wheel hubs differ from the Horch so circular rings were cut from 0.10 sheet styrene and inserted in the kit rims for detailing later. The fenders have been added and the rear superstructure built up.
A bit further along. Wheels added and the roof test-fitted. The roof was made by layering two sheets of 0.20 sheet styrene and glueing it to a slightly wider piece that matched the dimensions of the body. When the edges of the laminated pieces are beveled and sanded, it looks seamless.
More details added: luggage rack, rear doors, door handles and hinges, wheel bolts, front lights (made from the Horch's wheel hubs since the H 25 had larger lights than those in the Italeri Horch kit,) and windows made from thin sheet butyrate.
View from the rear. Doors temporarily attached with Blu-Tack, door handles and mechanisms in place, the interior nearly finished, and rear bumper under construction. The windows are covered with fine plastic mesh from the box labelled "Save this scrap of whatever it is because it might come in handy 10 years from now." It perfectly replicates the wire mesh used as added protection in deflecting shrapnel, small arms fire, and grenades.
Stay tuned for future updates