It could be either a leaky fuel tank or one of the many pipes and hoses that connect to the tank itself. The fuel tank of the 126 SD is behind the back seat.
If you search the archives for "fuel leak" or "fuel smell" you should get some matches.
On my '83 SD the hose adjacent to the fuel filler had pulled off. I'm surmising that at least once during filling the tank some excess fuel ran out and into the trunk rather than through this hose and onto the ground.
To get to this hose I had to remove the side and front trunk liners and remove several screws from the steel bulkhead that separates the trunk area from the fuel tank. I had to climb into the trunk to get sufficient leverage to push the hose back on. There are lots of vent lines and fuel return lines that could be the culprit also. My leak was bad enough to soak the passenger side trunk liner with diesel fuel.
Robert W. Roe
1984 300SD 164K mi
[This message has been edited by Robert W. Roe (edited 08-01-1999).]