Know-How Exchange : Advertising/PR

To make front-end decisions, I typically recommend the 'objective-and task method':
1) determine what a desirable sales level is for your product (eg 10% of the market)
2) determine how many more people need to prefer your product to achieve (1) (eg 15%)
3) determine how many people need to be aware of your product to achieve (2) (eg 60%)
4) determine how many times the people in (3) need to be exposed to your message. For instance, 3 exposures appears ideal in many situations: the first makes the consumer familiar with your product, the second arouses interest/preference, the third induces action.

This will give you an estimate of the needed points of contact. Next, you will have to guesstimate how effective communication channels are and how important synergies among them are in your industry. For automotive retail, as for many products, I would say that ads (GRPS) are not enough: radio is often very effective in conjuntions with ads (synergy), while DM is often needed to get interested consumers to actually contact your company. In contrast, simply increasing ad frequency (what your agencies apparently propose) is less likely to give you more sales...