You can order NMO mounts with longer cable. Problem is the connectors are not usually installed so it can be cut to length.
One option is to look for the Larsen NMO mounts with the "FME" style connectors. There are two options:
1. The FME jack is on the antenna base at a 90º angle. You install the cable separate from the mount.
2. The NMO mount has a cable with the FME on the end.
The function of the FME connector is that it's a small connector, just a bit bigger than the coax, so it can be pre-installed and routed through the vehicle easily. When you get to where you are going with it, you install an FME to BNC adapter on the end. It's just slightly longer than a crimped on BNC.
Many years ago we ended up with a "pre-engineered" product that had one of these. It was the first one I'd used, so I was interested/skeptical. Anyway, it's worked fine, no issues.
You'll pay a premium for this, but it's not a lot. If you are not comfortable crimping/soldering your own connectors, or just don't have the tools, it's a good option.
Alternately, if you were to tell one of us exactly what you wanted, we could just build it for you.