My dad wants to cosign a car loan for me. but doesnt know if this will affect his alimony that he pays to his exwife

Your father's alimony payments will not be affected nor will his ex-wife have a claim to this automobile.  In fact, the bank that you borrow from will often times own the vehicle until you have made all your payments, at this time you inform the bank whose name you want on title.  In the event your borrower allows you to be on title and just asks that your father cosigns, your father will be on the hook to make payments if you default, but he will have no interest (title) in the car. 

I am a lawyer in CT and practice int his area of the law.  Your dad co-signing on a loan does not subject the car the being taken to satisfy alimony payments.  the car is in your name as owner and thus, she cannot take the car to satisfy and obligation that is not yours.