Hi...I'm not a native speaker but perhaps I canhelp you. In my native (Slovenian) language we have the same expression. It only indirectly implies that the truck's condition was getting worse - it's basic meaning doesn't stress the process (of getting worse) but rather a present state compared to a past state. so what the man wanted to say in that case was that the truck is (at present) in a bad condition, but this used to be different in the past. He wanted to say that once the window worked, i.e. that it hasn't been broken for ever.
In Slovenian, we often use this expression when we refer to the condition of a building, for example:
A: 'The old school is dilapidated. Nobody wants to have it renovated.'
B: 'Yeah, it has seen better days...' = it wasn't always in such a poor condition