Products with Vehicle Fit Guide I'm in the process of developing a website for vehicle cargo boxes and adventurous vehicle accessories, and the client has requested a fit guide.

What I need is a jumping off point:  Do I build a taxonomy list of vehicle makes/models/year first and then ask vendors such as Yakima and Thule for their products? Do I ask the vendors first for their products and incorporate the vehicle list later?

I'm ultimately looking to host all vendors products within the site itself, and not have any iFrame solutions or link to vendor cart solutions.

I've searched for days now and have found multiple pre-built databases for vehicle's but am still wondering where to start...
Start wherever it makes sense to you. Your project sounds iffy if your not hired directly by any of these entities to promote their products.

I'd recommend a content type or entity as Accessory and then taxonomy terms for Make Model and Years of cars which will facilitate searching.

Start with that site structure, if it doesnt work for you then change it later.