Easy.
The truck probably has their phone number and some other identifying information. You google it, find out the person in charge of truck lettering and request to talk to him/her.
When connected, you introduce yourself and tell them you're only going to take two minutes if their time. Then you simply tell them you have some information you'd like to share with them that you believe you can help in their advertising on trucks... and possibly other areas.
If he/she simply say not interested.... you thank them and tell them you will still send them some information.
What did you lose ?? A few minutes googling and a five minute phone call.
If you find enough businesses... say 8 or 10 a week.... you're talking maybe an hours worth of your time at most, but if one comes through.... you might just land a $1,000 month contract..... a $25,000 a year customer and multiply that by however many weeks a year you wanna do that......
If you land a big one.... once a quarter, you'll be flying in no time. Get a few other small ones in there and you're gonna be making a nice living.
Believe me, I still do this and it works.
When you belong to networking groups, you become real good at talking other professionals into this kinda stuff. It's not always based on price, but how you hold yourself and how knowledgeable you come across. Can this multimillion dollar company trust you with their fleet of trucks, their weekly sign orders or any other stuff they might muster up ??
This is exactly what the radio-guy did to you. He went down a list of sign shops and will continue until someone says... yeah, I want to be associated with these guys... I'll do it for 6 months.
So, you go down your lists of clients you want and start pounding the streets.
Just don't be annoying. Don't call back...... and call back...... and call back if you get some routine brush-offs or people obviously ignoring you. However, I've always said, there's enough work out there and enough to go 'round.... you just have to physically go after it and make YOUR business work, by working it. You will get rejected and this place s101 is a perfect example of how to take your kicks and jabs, not get upset and go right back for what you want. Don't be discouraged if people tell you they're going to report you or just tell you to get lost.... they're not the kind of people you want anyway. You want people like yourself, to make everyone happy.