Do you recommend the tow companies to your customers? Business goes both ways.
The way I handled this at my shop was to start by asking around. If other shops are recommending certain tow companies it means they have a working relationship and/or they are good companies. In my area I only have one consistently reliable company so that's who I recommend because I want my customers to have a great experience. They don't bring me a ton of business but that's OK, that's not as important as taking care of my good customers. I can trust that they won't talk a customer into going to another shop and they show up when they promise.
If you're wanting a quid-pro-quo arrangement, start by finding a company that has a similar set of core values and principles as you do. Bonus points if that's a company no other shop recommended yet. Now, offer to make them your exclusive tow company provided they communicate and prioritize your customers and check in with them on a regular basis. People still do business with people they like.
Get to know the drivers! Chat with them for a minute or two when they drop off a car. Help them unload it. When a customer asks for a recommendation, they are asking the drivers, not the owners. Buy them lunch or invite them to a cook-out when you have a customer appreciation day at your shop or stop by their place one morning and bring breakfast from some place. I have heard food is a strong motivator in our industry! 🙂