The D3 I bought second hand did not come with the antenna base plate. The manual states this metal base plate is needed when installing the antenna within the car. it also says not to alter the antenna base in any way otherwise it could alter reception.

Could I use any piece of steel of approximately the same size and shape or should I try to get one from pioneer?

much appreciated

You can use anything metal.

most excellent...thank you sir

if you think about it, its not used when mountion the antenna on the roof of the car, and no two car foors are the same, different sizes and different shapes (the amount of curve itself)

hell, if you mount it under the dash you can mount it on top of any metal under there (as long as there is no metal above the antenna, and there is not too much material for the signal to get through)