Why will my 2005 pacifica heat up only in idle not while driving

Possibly low coolant level. Check the coolant level in the overflow bottle, should be a mark for "cold" and one for "hot." If the engine is cooled down, the fluid level should be at the 'cold' mark. If the engine is running at temp, fluid level should be at the 'hot' mark.

It will only heat in idle bit when I start driving it goes back to the cold mark

Coolent is up

When the engine is running at temp, the coolant should be at the hot mark on the overflow bottle. If it's at the cold mark it is too low.

I think hen you are driving the air going through the rad is enough to cool it down but it probably gets hot as you set at a red light same as setting in your driveway, It sounds to me like your thermostat is only opening a little when engine gets to temperature [approx. 195 or 197 degerees depending on which thermostat you have. Therefore a thermostat is not very expensive or hard to change. I would say that is a good starting point. Second maybe you need a flush and fill the rad could be plugged therefore not cooling down enough when setting still. Hope this helps you. Good luck and have a good evening

so there is nothing in the overflow bottle how do I get coolant in the over flow bottle

Remove the cap on the overflow bottle (aka reservoir), and fill it with the appropriate mix of coolant to the "cold" line, after the engine has cooled down. Most antifreeze comes pre-mixed. Try not to overfill it but don't worry if you do, there's an overflow tube from the reservoir that will dump excess coolant to the ground. Keep checking it after running the engine a few times to make sure you're not still running it low on coolant.

If there is no cap on the over flow there should be a hose besides the one going to the rad it will run to where ever you fill the overflow bottle from sometimes they are not on the tank