Making a complaint about a historic motor finance agreement is not itself a new credit application, a missed payment, or a default, so it does not typically affect your credit score. You're simply querying whether a past agreement was arranged fairly, not applying for new credit or missing any obligations on an existing one.
In more detail
What actually does affect a credit score?
Credit scores are generally influenced by things like new credit applications, missed or late payments, how much of your available credit you're using, and the length of your credit history. Raising a complaint about how an old agreement was arranged doesn't fall into any of these categories.
Could redress itself change anything on my credit file?
If a lender agrees your agreement was affected and adjusts the record as part of resolving your complaint, for example, correcting historic interest charged, this is a correction to accurate reflect what should have happened, not a negative mark against you.
What if the agreement in question has already been settled or the car sold?
Since the agreement is already closed, raising a complaint about it doesn't reopen it as an active credit line or create a new entry on your file.
Should I be worried about lenders treating me differently for complaining?
No, making a legitimate complaint about a past agreement is a normal consumer right, and it isn't something lenders are permitted to hold against you in future dealings.