You gave up all of your rights when you showed them the weed and the bowl in the first place.

You waive your right to the 5th whenever you admit or allow anything, especially when you show them both the drug and the paraphenelia in the first place. Now, if you guys would have simply placed the bowl and weed under a car seat or in a glove box, then the Plain View Rule comes into play. If they cannot visibly see anything out in the open, and you give them no consent for a car search, they CANNOT under ANY circumstances search your vehicle. They can order drug dogs in, but most likely you guys would have just gotten bitched at and then ticketed for whatever was the original reason you got pulled over for.