Would suggest this if this is your first company vehicle with a wrap to make the subject of the wrap your most profitable product or service. Select one of the three you listed, signs, wrap or t-shirts. Would select signs because the other two will follow from the wrap job naturally in conversations. Then would suggest making the background the best design you offer in that genre with the thought in mind that it will be the support for the message and not detract from the message. Definitely agree that you should make the background yourself. People will ask about it and you making it is important to the making you an expert in their minds. Agreeing with rjssigns on hooking the eye. Now pick a font that is unique but very readable from a distance for the product/service you are offering and contact info. Finally add your company name for those who can better remember your name and can't remember the contact info or those who will want to tell others.
Current design hits
Bright and catches the eye.
Current design misses
Copy Hard to read from a distance
Subject of wrap is too small
Too many subjects offered
Back ground distracts from copy.
Here's something of a first draft for an example.
To critique my own it may not be your market and the back is not done. You might make the background with brushes and other signs making tools embedded covertly in the design. This one upon close inspection has men and forest features in the background. It's very funny but some folks do close up inspections and find the covert stuff and love the surprise. Also it gives them something to say to others about subtleties and quality of your attention to detail.
Of course this is just an opinion and it is offered with good intention as help. Take what you wish and leave the rest.
