If I need increasingly stronger fragrance every month to make the same SUV smell pleasant, should I investigate accumulating odour sources and sensory adaptation before buying something stronger?

Yes, investigate both — and start with the one piece of evidence you already have and have probably not looked at properly, which is not the level you have reached but the gap between the increases. Think back over the last few months and try to date them. If the increases are spaced evenly, a growing source is plausible and worth hunting. If the gap has been shortening — three months, then two, then six weeks, then a month — you are looking at a loop rather than a problem, because nothing inside a vehicle accelerates in order to keep pace with your purchases. A nose does, and here is why. Adaptation is not set against zero; it is set against habitual exposure. Whatever level you live with becomes the level at which you stop noticing, and it becomes that quite quickly: adaptation to a fragrance is close to complete within about twenty minutes of getting in, and adaptation to a background in a vehicle you own takes days rather than months. So every time you increase, you raise your own habitual exposure, your threshold follows it upward, and the amount of time before the new level stops registering gets shorter. More, then sooner, then more again. The shortening interval is the signature, and it is the single most useful thing on this page. Two more facts make it worse for you specifically. The driver adapts fastest of anybody in the vehicle, because the driver is closest to whatever is producing the scent, exposed for the longest, every day without a break — so the person making the purchasing decision is the person least able to judge it. And the two honest instruments available are both outside your own habituated judgement: the first second of opening the door after a closed night, before your nose recalibrates, and a passenger getting in for the first time with no history of the vehicle at all. Use the passenger. Ask for one word, take the word, and do not negotiate with it. Now the way out, which is stopping rather than switching. Switching gives you a few days of novelty because an unfamiliar scent registers again, and then your nose files it alongside the last one and the loop resumes at the higher level you had already reached — which is exactly how households end up with four half-used bottles and the same complaint. Stopping is different in kind. Take everything scented out for three to five days. Days one and two are genuinely unpleasant, and I would rather warn you than have you abandon it on the Tuesday: you will meet your own vehicle honestly for the first time in a long while, and you may well find a source you had stopped being able to hear. By the third morning the threshold has dropped noticeably; by the fifth, things you had stopped noticing are obvious again. Then restart with one product, in the mild hanging format, at a level that feels slightly inadequate — and hold it for a month without touching it. It will not feel inadequate in a fortnight. The concession is that opening stretch, which is the only cure in this whole cluster that feels worse than the disease for forty-eight hours. And the line underneath everything: fragrance will not remove this. If the days with nothing turn up a real source, that source has to go, and no product we sell will do it. Intensity is a shared comfort decision best judged by a passenger climbing into the back row rather than by the driver, and nothing here is a claim about driving, alertness or road safety.