It is bolted to the exhaust manifold, rear of the seconday air valve (don't mistake the 2). Some 104 engines had their EGR updated due to sticking problems -- marked with a red dot.. so look for it.
To test valve apply vacuum to it at idle and if car stumbles/dies the valve is sticking and may need to replaced or cleaned. If nothing happens then your EGR tube is likely blocked with carbon and needs to be cleaned/replaced.
You can credit Arthur Dalton for this information -- he has written extensively on this topic.
Good Luck
