There is no custom fields for dealer. However you can add custom field to ads as select box. Users will have to choose if they are posting an ad as dealer or user.
If you mean adding custom field to dealer itself like address, opening hours, phone, etc. Then those fields can be entered as plain text to info field on dealer edit page. To see info field navigate to “Users” page in admin panel and click Edit button for any user. Then on “Permission” drop down choose “Dealers” and you will see Website, Logo, Info fields for dealer.