On my old 04 plate citroen relay I could just splice into the return to tank fuel line. The return pipe went all the way to the bottom of the tank.
Then there was a change in eu regulations for manufacturers and this fuel line now wasn't to reach to the bottom of the tank.
On my 62 plate Peugeot Boxer, I had to remove the fuel pump, drill and fit a connector/adaptor with a pipe down into the tank, and connected my heater to that. I had to be careful how I routed the pipe to the bottom of the track so it won't interfere with the fuel level sender unit. That pipe also wasn't too go to the bottom of the tank.
My van has an inspection cover plate inside the driver's cabin floor. So I didn't need to drop the tank. Not all vans have this inspection cover. In that case, the tank has to be lowered/removed.
The other part of this new eu regulation for fitters was that they weren't to drill a hole in any part of the tank that was plastic to fit the standpipe. They could only do this if the tank was made of steel.
This is a bit of a minefield.
There is/was a video on YouTube of someone showing how to fit a standpipe into the plastic tank of a Citroen Relay by partially dropping the fuel tank. He also made a scoop to collect the plastic cuttings from the drilling process so those those didn't drop into the tank. He removed the fuel pump to do this.