Car smells disgusting, how do I fix it?

It is very difficult to diagnose smells without actually getting a clear description. I have smelled some pretty bad ones before. I had a customer leave salmon under the back seat and that really was disgusting. Sometimes rodents will crawl inside the cabin and build nests as well. They sometimes die inside, and the stench is unbearable.

One way to check for rodents is to remove the cabin filter or if you remember from the last cabin filter change if there were droppings or maybe they were chewing at the filter? If you have a water leak inside because the A/C drain hose has a spider nest inside can also cause it to smell if it has been leaking for a while or water got inside.

If that is the case, you could have mold. For that you would need to contact a mold remediation company.
That job is very expensive, and I have seen insurance companies just do a total loss on a vehicle because the repair is so expensive. My advice would be to have it looked at by a repair shop to find the root cause of the odor.

I have attached a picture of a vehicle that had mold. We had to scrape the insulation after stripping the interior down. Very big job. Very expensive. After the mold treatment we had to install new insulation and replaced lots of parts.

I had updated my original post because I forgot to upload a picture of a vehicle we took apart.

This smell can be caused by a leaking or seeping heater core, can you please check the passenger's side floorboard to see if there is coolant present? If so, you have found the problem.