To protect individual vehicles you may want to consider using T-Rex snap traps beneath the vehicle and resting on the vehicle tires overnight. Tamper-resistant rodent bait stations loaded with bait such as Fastrac Blox will help reduce the level of mice and rats if there are more on the property to a more manageable level. These products can be purchased together in our Rodent Kits: http://www.domyownpestcontrol.com/rodent-kits-c-21_405.html.
It sounds like you are treating for mice in your home. For rodent control with a product like Ramik Green Bait Packs, you want to maintain a fresh supply of bait as long as you are seeing live rodents or signs of them such as new droppings or chew marks. Bait should always be used in a way which is secure, such as in bait stations. Indoors, this keeps the bait in place so that the rodents do not just carry it back to their nest and hoard it. Stations also protect bait from non-target animals and children. Outdoors, stations protect bait from weathering, from being relocated, and from non-target animals like wildlife and pets.
We are more tips in our Mouse Control Guide here.
Ramik Green Bait Packs start killing rodents within 4 to 5 days after taking a lethal dose.