When a fuse blows, there is a reason. A direct short circuit would blow either a 15 ampere or a 20 ampere fuse immediately. However, when you put in a 20 ampere fuse, and it doesn’t blow, while a 15 ampere fuse does blow, you are pulling more current through the wires than the wires were intended to carry. This leads to heat, melted insulation and a possible fire. On a fused circuit, a dead short rarely causes a fire. However, pulling more current (amperes) on an overfused circuit (using a 20 ampere when a 15 ampere is required) sets up the conditions for a fire whether it is in a car or in a house.
I bought a new Ford Tempo in 1985. I had only had the car a month when the fuse blew for the tail lights. The dash lights were on the same circuit so I knew when the fuse blew. A replacement fuse would last up to a week and then would blow. The car was on warranty, so I took it back to the dealer. The problem turned out to be that some Ford Tempo models were equipped with a tail light monitor. This monitor circuit had a very thin wire leading back from the tail lights. My Tempo didn’t have this feature. A goof at the factory had this thin wire powering the tail lights. The insulation melted off this wire and when it touched the body of the car, the fuse would blow. The service technician thought I was fortunate not to have had a fire in the trunk. At any rate, the proper wire was connected and the problem was solved.