I can name only 3 instances where people actually had the privilege to be in Columbo's car.
1. Steve Schirripa gets to sit in the car in Columbo Likes the Nightlife (2003) 2. Grant Heslov (if that's the name) gets to ride with Columbo so a traffic camera can take a picture in Murder of a Rock Star 3. Larry Storch is terrified in Negative Reaction (1974)
I think Donald Pleasance rode in his car on the way to jail in "Any Old Port in a Storm". Columbo presented him with a bottle of wine and a wine glass.
There's an entire scene in the convertible -top down- in the parking lot of a 50's style outdoor drive-in burger joint. Columbo questions Peter Hamilton (Leslie Nielsen), over cheeseburgers, on the alibi of his fiancee Beth Chadwick (Susan Clark). The waitress instructs Columbo to roll his car window half-way down, she attaches the tray of food to the outer window, informs Columbo that they are out of his onion rings, asks him if he would like to substitute ?'s and also reminds him not to drive off with the tray when they were through eating. One more thing... Steve Boccho co-wrote the episode. Oh, one more thing... Murder victim Bryce Chadwick was played by the actor who played Oscar Goldman in The Six Million Dollar Man.
Also Lawrence Harvey in "The Most Dangerous Match" when he's driving Emmitt Clayton back to the hotel but Columbo "just happens" to get hungry and pulls into the restaurant where Clayton and Dudek were playing chess on the tablecloth. Columbo had the top down in this episode also if memory serves me correctly.
Forgotten Lady. Columbo goes to his car and finds a friend of his, a cop, sitting in it who tells him that he needs to go to the shooting range - he's required to do so.
Murder of a Rock Star (1991) makes 11: The lab techician of the company that took the photograph leading to the speeding ticket. Columbo recreates the circumstances and takes him along.
Actually this episode may count as two instances because later on, a female cop wearing a mask sits in his car as Columbo confronts the killer and exposes his trickery.
In 'Undercover' (1994) one of the junior detectives does not just get to sit in the car but actually drives it to meet Columbo, who gets out of a rented limo after posing as a maffia guy.
In "Negative Reaction" he gives the driving instructor a lift back to his office when he's questioning him about a suspect who passed his driving test.
In "Catch me if you can" Columbo told the millionaire author that they should take his car, that it was, "FRENCH and RARE". The author cleverly ended up with him driving her Rolls.