Friend showed me a way, but unfortunately it's not a glitch. It's "easy" provided you have a friend that has a magic menu.
1.Friend with magic menu. (can't emphasize this enough)
2. Office Garage and Office Custom Auto Shop
3. Lifeguard blazer has to be stored(already has a tracker) in Office Garage.
Not required, but convenient to confirm everything worked:
Place Blazer Lifeguard in spot #6 of your Office Garage
How to insure:
1. Drive Blazer Lifeguard out of garage.
2. Get out of the Blazer.
3. Go back to the same garage the Blazer Lifeguard is stored.
4. Once inside garage, personal vehicle icon will be outside.
5. Your magical friend spawns a cheap car that is store-able, somewhere on the first floor is good. (I recommend a Rusty Rebel).
6. Get into the Rusty Rebel and drive into Office Custom Auto Shop.
7. Add insurance (you'll be surprised how low the price is...)
8. Exit Office Custom Auto Shop menu and return to garage.
9. Your Lifeguard Blazer is now insured.
Note: this seems to work with any car that is in garage/has tracker on it...
I'm sorry this isn't just a 100% glitch that can be used right away but at least it's a way.
Hope this helps whoever has that kind of "magic" friend.
Wish there was a way to give cars to friends so I could to this with Insurgent... :l