Should I clean the cargo area after long trips before judging whether my normal car perfume has stopped performing?

Yes, and the reason is a measurement problem rather than a cleaning preference. Nobody ever experiences a fragrance on its own. What you experience is a fragrance plus whatever else the cabin happens to be doing at that moment, and your impression is the relationship between the two rather than a reading of either. That has a consequence people rarely follow through: if the background changes, the same product reads differently even though nothing has happened to the product at all. Now consider what a long trip does to the background. A week of material arrives in one load, pressed into the cargo carpet under the weight of the cases for five or six hours, and fabric that has taken something up hands it back slowly over the following days. So on the Monday after you get home, the background in your vehicle is not merely higher than usual — it is actively falling, which means every reading you take is taken against a different reference point from the last one. Judging a product in those conditions is like weighing something on a scale that somebody is leaning on. It produces a number. The number is about the leaning. This explains the pattern I see most often in letters: somebody decides on the Monday that their usual scent has gone flat, buys something bolder on the Tuesday, and by the following weekend has a vehicle that reads heavy and confusing, with two products in it and the original one having been perfectly adequate all along. Fragrance will not remove this. A car perfume adds a scent to the cabin and has no mechanism for taking one out, so adding a second thing on top of a falling background never gives you clarity; it gives you a mixture at the exact moment you most needed to be able to read the cabin. The sequence that sorts this out is short. Unload completely. Lift the cargo floor panel and look in the well beneath it. Take the mats out of the vehicle entirely to clean and dry them. Work the cargo floor, the tailgate strip and the folded-seat channel with a crevice tool. Damp-cloth the hard surfaces. Give it an hour with the doors open in shade. Then take three morning readings, after closed nights, before you switch anything on — and give your verdict on the fourth day. The cost is one postponed decision. The benefit is that the decision is about the product. Two standing notes: intensity is a shared comfort decision best judged by a passenger climbing into the back row rather than by the driver, and nothing on this page is a claim about driving, alertness or road safety.