I'm sorry about the situation you're in. I hope you can find the help and vehicle you need, that said, you give us little details, and the details you give show some lack of knowledge (at least how it's presented) on car buying.

First off, why is your current vehicle unsafe? What is your current vehicle?

How much car can you actually afford? You say you can afford $400/mo payment, but that doesn't mean anything as dealers can and will take advantage of you. A dealer can easily put you into a cheap car with a high interest rate that will eat up your $400/mo and end up costing you big money long term that will end up hurting you more financially. Don't look at purchases as monthly payments, look at the total cost. Also, since you say you can afford up to $400 a month, does that include insurance? Fuel? Maintenance? You also mention "Dealers that have low down payments". Predatory lenders can offer little to no down payment in exchange for a loan with astronomical interest rates that will ruin you financially as mentioned above. How much do you have set aside for a down payment?

What price range are you shopping for? $10k? $20k? More? What kind of truck are you looking for? Affordable trucks in this market are extremely hard to find. How long are you planning on keeping the truck for? Does your side work require a truck? Can you get a work van instead?

These are all very important questions to answer before even thinking of stepping foot into a dealer, especially smaller independent or "Buy here pay here" type lots. They'll see you coming from a mile away and take you for everything.