Relays are used to switch power on to devices that require a fairly high amount of current. The blower motor uses perhaps around 8 to 12 amps. The relay is a fairly small device and if you do a web search on a Bosch relay you will be able to see what they generally look like. The blower relay should be located somewhere under the dash. They unplug from a socket so they are easy to replace. Once you find the relay try tapping on it to see if the blower will turn on. If it does then replace the relay, it has bad switch contacts inside it.