You are not liable for his drugs or other criminal behavior unless you participated in them with him or knowingly helped him hide the drugs, etc. So legally, if not yours, you are not responsible for this and should not face liability. People are only responsible for their criminal behavior.Practically, the issue is showing that the drugs were not yours and also that you did not know that he drugs on him but nonetheless helped him transport them or conceal them. The police only know what the facts and evidence show or what people tell them. The drugs were under you seat; that makes it appear they were your or that at least you knew they were in the car and were helping transport them. If your associate tells the police that these were your drugs, or tries to get leniency for himself by saying you were buying, selling, transporting, etc. them, it may be hard for you to prove otherwise. You are strongly advised to retain a criminal defense attorney to help you.